How to Sail on Sunlight

Solar sailing is a revolutionary way of propelling a spacecraft through space. A solar sail spacecraft has large reflective sails that capture the momentum of light from the Sun and use that momentum to push the spacecraft forward. The Planetary Society’s LightSail 2 mission is one example of this technology in action.

When was the solar sail invented?

Solar sailing is a concept with a long history, dating all the way back to an idea Johannes Kepler shared with his friend Galileo Galilei in 1608.

The Planetary Society is one of the pioneers of solar sailing technology. In the early 2000s, we built the Cosmos 1 solar sail spacecraft. On June 21, 2005, Cosmos 1 launched from the submarine Borisoglebsk in the Barents Sea. However, a rocket failure prevented the spacecraft from reaching orbit. Had the mission been successful, it would have been the first ever use of a solar sail spacecraft, as well as the first space mission by a space advocacy group.

What is a solar sail made of?

Current solar sails are made of lightweight materials such as Mylar or polyimide coated with a metallic reflective coating. LightSail 2 uses 4 triangular Mylar sails that are just 4.5 microns (1/5000th of an inch) thick. They unfold using 4 cobalt alloy booms that unwind like tape measures. The sails have a combined area of 32 square meters (344 square feet), about the size of a boxing ring.

The Planetary Society’s LightSail 2 spacecraft sits on its deployment table following a successful day-in-the-life test at Cal Poly San Luis Obispo on May 23, 2016.

How does solar sailing work?

Light is made up of particles called photons. Photons don’t have any mass, but as they travel through space they do have momentum. When light hits a solar sail—which has a bright, mirror-like surface—the photons in that light bounce off the sail (i.e. they reflect off it, just like a mirror). As the photons hit the sail their momentum is transferred to it, giving it a small push. As they bounce off the sail, the photons give it another small push. Both pushes are very slight, but in the vacuum of space where there is nothing to slow down the sail, each push changes the sail’s speed.

Why do we want solar sails? Where will solar sails take us?

The most exciting thing about solar sails is that they could open up new avenues for space science and exploration. A solar sail-propelled spacecraft could reach distant planets and star systems much more quickly than a rocket-propelled spacecraft because of the continual acceleration that solar sailing provides. The technology for interplanetary or interstellar solar sailing is still far from being developed, however.

In the near-term, solar sailing can also be used effectively for other classes of missions including solar monitoring, multi-object flybys, and “pole-sitting” spacecraft for continuous observations of Earth’s or another object’s polar regions.

Solar sails can also provide propulsion for CubeSats—small, inexpensive satellites that are increasingly being used by emerging spacefaring nations, small companies, and even school groups—allowing them to maneuver in space without relying on rocket fuel. The Planetary Society’s LightSail mission is demonstrating the potential use of solar sails for CubeSats.

Who has built solar sails? What solar sailing missions exist?

Since the failed Cosmos 1 mission, solar sails have been successfully built and launched by the Japanese Aerospace Exploration Agency (JAXA) with their IKAROS spacecraft that first demonstrated controlled solar sailing, by NASA with their NanoSail-D spacecraft, and by The Planetary Society with our LightSail 1 spacecraft.

Many more solar sailing missions are in development, including The Planetary Society’s LightSail 2 mission and NASA’s NEA Scout mission to a near-Earth asteroid.